Download Drivetrain

The drive system consist of a six-wheeled rocker-bogie suspension design initially
develop by the Jet Propulsion Laboratory for the Mars expeditions. The term “bogie”
comes from old railroad systems. A bogie is a train undercarriage with six wheels that
can swivel to curve along a track. The term “rocker” comes from the design of the
differential, which keeps the rover body balanced, enabling it to “rock” up or down
depending on the various positions of the multiple wheels.1
The rocker design ensures all six wheels will remain on the ground while driving over
rough terrain and that the rover body only goes through half of the range of motion that
the “legs” and wheels could potentially experience without this suspension system. The
rover rocker-bogie design also allows the rover to go over obstacles (such as rocks) or
through holes that are more than a wheel diameter in size.
Each wheel has cleats, providing grip for climbing in soft sand and scrambling over
rocks. Each of the wheels also has an internal motor to drive it. Additionally, the four
outer wheels are articulated by servos. This steering capability allows the vehicle to turn
in place, a full 360 degrees. The 4-wheel steering also allows the rover to swerve and
curve, making arching turns.
The rover is designed to withstand a tilt of 45 degrees in any direction without
overturning. However, the rover is programmed through its “fault protection limits” in its
hazard avoidance software to avoid exceeding tilts of 30 degrees during its traverses. The
wheels are designed to encase the motors internally to allow for added ground clearance.
The motors are planetary gearbox 20-volt DC motors. They have approximately 450 inounces of torque at 35 RPM, which will give the rover approximately nine inches per
second travel rate. It also gives the wheel approximately 12 pounds of force at the outer
diameter. They have an operating temperature of -20º Celsius to 125 º Celsius.
The motors are controlled by Darlington transistor and are capable of being reversed
through a double-pole double-throw relay. The suspension is connected to the frame via a
central spring and damper system. The drive system is controlled by two Motorola
68HC11 processors that communicate through a serial peripheral interface. The main
processor controls all the higher-level collision and obstacle avoidance and the slave
processor generates the pulse width modulation for motor speed control and servo
actuation.
A microcontroller (MCU) is a computer-on-a-chip. It is a type of microprocessor
emphasizing self sufficiency and cost effectiveness, in contrast to a general-purpose
microprocessor (the kind used in a PC). A microcontroller is an entire computer system
within a single chip, usually containing the central processing unit (CPU), memory
(RAM, ROM, EPROM, and EEPROM), input and output (I/O) ports, timers and
counters, analog to digital (A/D) converters, and communications capabilities.
In contrast to general-purpose CPUs, microcontrollers do not have an address bus or a
data bus, because they integrate all the RAM and non-volatile memory on the same chip
as the CPU. Because they need fewer pins, the chip can be placed in a much smaller,
cheaper package. Integrating the memory and other peripherals on a single chip and
testing them as a unit increases the cost of that chip, but often results in decreased net
cost of the embedded system as a whole. (Even if the cost of a CPU that has integrated
peripherals is slightly more than the cost of a CPU + external peripherals, having fewer
chips typically allows a smaller and cheaper circuit board, and reduces the labor required
to assemble and test the circuit board). This trend leads to design.
A serial peripheral interface (SPI) is an interface that enables the serial (one bit at a time)
exchange of data between two devices, one called a master and the other called a slave.
An SPI operates in full duplex mode. This means that data can be transferred in both
directions at the same time. The SPI is most often employed in systems for
communication between the central processing unit (CPU) and peripheral devices. It is
also possible to connect two microprocessors by means of SPI. The term was originally
coined by Motorola. National Semiconductor has an equivalent interface called
Microwire. Serial interfaces have certain advantages over parallel interfaces. The most
significant advantage is simpler wiring. In addition, serial interface cables can be longer
than parallel interface cables, because there is much less interaction (crosstalk) among the
conductors in the cable. Many types of devices can be controlled by an SPI, including
shift registers, memory chips, port expanders, display drivers, data converters, printers,
data storage devices, sensors, and microprocessors. Data is transferred serially over a
cable, input to a shift register, and transferred within each subsystem by means of parallel
processing.
In electronics, the Darlington transistor is a semiconductor device, which combines two
bipolar transistors in tandem (often called a "Darlington pair") in a single device so that
the current amplified by the first is amplified further by the second transistor. This gives
it high current gain (written β or hFE), and takes up less space than using two discrete
transistors in the same configuration. The use of two separate transistors in an actual
circuit is still very common, even though integrated packaged devices are available. This
configuration was invented by engineer Sidney Darlington. The idea of putting two or
three transistors on a single chip was patented by him, but not the idea of putting an
arbitrary number of transistors, which would have covered all modern integrated circuits.
A Darlington pair behaves like a single transistor with a very high current gain. This is
beneficial as many commonly used transistors with high gains have a low current
threshold. The total gain of the Darlington is the product of the gains of the individual
transistors.
A typical modern device has a current gain of 1000 or more, so that only a tiny base
current is required to make the pair switch on. Integrated devices have three leads (B, C
and E), which are equivalent to the leads of a standard individual transistor.
The base-emitter voltage is also higher; it is the sum of both base-emitter voltages:
VBE = VBE1 + VBE2
For activation, there must be ~0.6 volts across both base-emitter junctions which are
connected in series inside the Darlington pair. It therefore requires more than 1.2 volts to
activate. When a Darlington pair is fully conducting, there is a residual saturation voltage
of 0.6 volts in this configuration, which can lead to substantial power dissipation.
Another drawback is that the switching speed can be slow because of the inability of the
first transistor to actively inhibit the current into the base of the second device. This can
make the pair slow to switch off. To alleviate this slowness, a resistor of a few hundred
ohms between the second device's base and emitter is often used. Integrated Darlington
pairs often include this resistor.
It has more phase shift at high frequencies than a single transistor and hence can become
unstable with negative feedback much more easily.
Darlington pairs are available as complete packages but can also be made up from two
transistors; Q1 (the left-hand transistor in the diagram) can be a low power type, but
normally Q2 (the right-hand transistor) will need to be high power. The maximum
collector current IC(max) for the pair is the same as IC(max) for Q2. A typical integrated
type of power device is the 2N6282, with a current gain of 2400 at a collector current of
10 amps and an included switch-off resistor.
A Darlington pair can be sufficiently sensitive to respond to the small current passed by
skin contact, and can thus be used to make a touch sensitive switch.
Figure 1: Drive System Block Diagram
Figure 2: Rocker-bogie Diagram